The purpose of this study is to determine whether the generic form of clozapine (Zenith) differs in bioavailability and pharmacodynamics from the Clozaril brand of the same drug. The study drug is used to treat psychosis and study subjects will have been stabilized on Clozaril. Differences in response to the two drugs will be measured in terms of cognitive tests, PANSS and CGI ratings, and ratings of dyskinesia and akathesia.